(a) Field of the Invention
The present invention relates to an improved, high strength, high loop density, woven back pin seam for use in joining the ends of papermakers' and like fabrics.
(b) Description of the Prior Art
Woven fabrics, intended for use in either the forming, pressing or drying sections of paper making machines, are usually rendered endless by one of three methods:
1) endless weaving, such as is described in U.S. Pat. No. 2,903,021;
2) joining the opposing ends of a flat woven fabric with a permanent seam, such as is described in U.S. Pat. No. 3,366,355; U.S. Pat. No. 3,596,858 or U.S. Pat. No. 3,700,194; or
3) joining the opposing ends of a flat woven fabric by forming small loops in the opposing fabric ends and then interdigitating these loop ends during installation of the fabric on the papermaking machine to form a passageway through which a pintle is inserted to form a hinge-type joint. Such seams are described, for example, in U.S. Pat. No. 4,182,381, U.S. Pat. No. 4,469,142 and U.S. Pat. No. 5,092,373.
The present invention is concerned with the last of these methods. Although several types of these seams are presently in use on papermaking fabrics, the most desirable type of seam, which produces the least mark in the paper web in contact with it, is a woven back pin seam, wherein the warps of the fabric are used to form the loops which receive the joining pintle. The loops are formed by weaving back the ends of some of the warps into a nearby warp path in the fabric, in registration with the fabric weave. Such seams are well known in the prior art, and are referred to in the trade, and henceforth in this disclosure, as pin seams.
Because of their length, dryer fabrics are almost always joined on the paper machine with an on-machine seam, and therefore this invention applies particularly to dryer fabrics. However, press fabrics are also often joined by pin seams, as are some coarse forming fabrics, and the invention applies equally well to these types of fabrics.
It is well known that most prior art pin seams are formed in fabrics woven in 4-shed or 8-shed weave patterns. Such designs are particularly well suited to pin seaming due to their even number of sheds. The pin seam is typically made by removing a predetermined number of weft strands from each end of the fabric and when reweaving the crimped warp strands, which now project from both fabric ends, through a plurality of added weft, in a manner that is well known. The weft strands are generally chosen from a group consisting of thermoplastic polymer monofilaments, spun yarns, multifilament yarns, plied monofilaments, or combinations thereof. A warp strand is typically folded back and interwoven partway into a nearby warp path until it reaches the warp strand normally residing in that path, which is also rewoven into the added weft strands. Both strands are clipped off closely to the surface of the fabric to provide termination points at various distances from the last original undisturbed weft strand in the fabric end. One-half of these folded back warp strands are bent around a loop-forming rod placed adjacent the last added weft strand to form pintle loops. The remaining folded back warp strands are bent around the last added weft to form non-load bearing loops. The same method is employed at the opposing fabric end so as to produce seaming loops which are identical to those made at the first fabric end. The seam is then closed by interdigitating the two sets of pintle loops, and inserting the pintle. It will thus be seen that only 50% of the available warp strands from each opposing fabric end are used to form the load bearing pintle loops in these prior art pin seams.
Prior art pin seams suffer from several disadvantages, including, but not limited to the following:
1) the difficulty with which the fabric ends are interdigitated and the pintle inserted during installation on the paper machine,
2) fabric failures due to insufficient strength in the region of the seam, and
3) marking of the moist paper web by the seam.
Ease of installation is a very important feature of pin-seamed fabrics. If a lengthy time is required to install a fabric on a large paper machine due to difficulties encountered during interdigitation of the pintle loops, or insertion of the pintle across the fabric width, then the cost to the paper maker in terms of machine down-time can be great. Numerous attempts have been made to improve the ease by which the seam is formed and the pintle inserted into fabrics which are rendered endless during their installation on the paper making machine; U.S. Pat. No. 4,035,872 and U.S. Pat. No. 4,945,612 provide examples of various methods devised to improve or assist the interdigitation of the opposing ends of the fabric, and simplify insertion of the pintle by providing a more open pin-receiving channel. U.S. Pat. No. 4,469,142, for example, discloses a pin seam having enlarged seaming loops with the objective of overcoming these problems.
It is well known that the seam is a weak point of the fabric, and seam failures are commonplace in all papermaking fabrics. Thus, it is also very desirable to provide a pin seam whose tensile strength is as near to that of the fabric itself as is possible. As previously noted, most prior art pin seams are made in 4- or 8-shed fabrics in which one-half of the fabric warp yarns are used to form pintle retaining loops at each opposing end of the fabric. This is equivalent to a 50% loop fill. The term "loop fill" is used henceforth to denote that percentage of the total available number of warp yarns at each end of the fabric which are used to form the pintle retaining loops. A 50% loop fill was thought to be necessary to permit the loops from the opposing fabric ends to interdigitate easily while providing an open passageway for the closing pintle, thereby reducing fabric installation time. Because the remaining 50% of the available warps are not load bearing elements in the seam, the tensile strength of such seams cannot exceed 50% of the fabric strength.
As used herein, warp fill is defined as the amount of warp in a given space relative to the total space considered. Warp fill can be over 100% when there are more warp strands jammed into the available spaces than the space can dimensionally accommodate in a single plane. Fabrics having a nominal warp fill of approximately 100% will generally have an actual calculated warp fill of from 80% to 120%, as do the fabrics of the present invention. Values over 100% are brought about by crowding and lateral undulation of the warp strands.
It is desirable that the seam not mark the paper which is being formed upon it. Seam marking can be caused in the dryer section by differential drying rates resulting from changes in air permeability in the seam area when compared to the body of the fabric, or by excessive pressure of any raised portions of the seam against the wet paper web as it is being held against a dryer cylinder. In any case, it is well known that a pin seam having relatively short pintle retaining loops, which is closed with a pintle of the proper size, will reduce any marking tendency. In general, the seam should provide as little difference as possible, with regard to both air permeability and thickness, When compared to the remainder of the fabric. A compromise between the requirements of non-marking and tensile strength is often required in order to provide a seam which can be quickly and easily installed in the fabric on the paper machine.
Numerous means have been proposed to optimize the above noted seaming requirements Of non-marking, strength and ease of installation. For example, Lees, in U.S. Pat. No. 4,026,331, discloses a woven back pin seam for use in single layer forming fabrics having warp fill greater than 86%. The patent teaches that seam marking may be reduced by selecting appropriate fabric weave structures and yarn diameters which will ensure that the thickness of both the fabric and seam are approximately the same. The seam is formed by unweaving the opposing fabric ends and then when reweaving folding back the loop forming warp yarns so that their crimp is in registration with the fabric crimp pattern. However, it is disclosed that this latter requirement restricts application of the method to symmetrical weave patterns (col. 4, lines 47-56). The seam utilizes 50% of the available warp yarns to form the load bearing pintle loops, thus its tensile strength cannot exceed 50% of the fabric tensile strength. The patent is silent with respect to the angular orientation of the seaming loops.
In U.S. Pat. No. 4,991,630, Penven discloses a 100% loop fill pin seam for use in single warp layer woven press felt base fabrics. The pintle loops at the opposing fabric ends are formed so as to be oppositely inclined to one another. heatsetting the fabric will allegedly then cause the pintle loops to be realigned so as to take on a substantially orthogonal orientation with respect to the pintle, thereby permitting easy seam closure. Those skilled in the art will realize that the 100% loop fill seam disclosed by Penven, as well as the prior art shown in FIG. 1 of the patent, can only be achieved if the warp fill of the fabric is less than 50%, otherwise there will be insufficient room at the seam to intermesh the pintle loops. Both FIG. 1 and FIG. 4 of Penyen show fabrics which appear to have a low warp fill. In contrast, the warp fill of the fabrics of the present invention must be from about 80% to about 120%. Therefore, the Penven disclosure and prior art are not relevant to the present invention, although the 100% loop fill seam is a desirable goal. Research by the present inventor has shown that it is possible to form a high loop fill seam for use in a high warp fill fabric without the attendant disadvantages of the prior art noted by Penven.
Prior to the present invention, manufacturers of paper machine clothing were unable to produce reliably woven back pin seams in fabric designs having odd numbers of sheds, such as 3-shed designs, and integral multiples thereof, such as 6-shed designs. MacBean, in U.S. Pat. No. 4,438,789, describes a pin seam in which 662/3% of the available warp strands are used to form pintle retaining loops in a high warp fill fabric having a 6-shed, semi-duplex, asymmetrical weave design. This patent recognized the difficulty of interdigitating a 662/3% loop fill seam and sought to solve the problem by forcing the loops into an erect position by means of supplementary multifilament yarns which are interwoven around pairs of warp loops, forcing them together into an orthogonal position to improve loop alignment, and to facilitate loop interdigitation. The main features of this patent are as follows:
i) the seam may be formed without rotating the warp yarns at the loops (col. 3, lines 15-17), PA1 ii) all of the projecting warp strands forming the pintle and retaining loops are woven back without regard to their pre-set crimp configuration or the crimp pattern of the fabric (col. 3, lines 24-26), and PA1 iii) selected pairs of pintle loops are grouped to form tandem loops with intervening retaining loops so that two-thirds of the available warp strands are formed into pintle loops (col. 4, lines 21-27), which are then drawn together by means of supplementary flexible strands to facilitate their. intermeshing and pintle insertion (col. 4, lines 28-36). PA1 i) high tensile strength, approaching that of the fabric, PA1 ii) low profile, so as not to mark the web formed thereon, and PA1 iii) ease of installation on the paper machine. PA1 a) the warp yarns are polymeric monofilaments woven at a warp fill of from about 80% to about 120%, and, PA1 b) the warp yarns from which the pintle retaining loops are formed at the first and second fabric ends are rewoven into the fabric so that their preset crimp is maintained in registration with that of the fabric weave pattern, PA1 i) the pintle retaining loops have a loop fill greater than 50%, PA1 ii) the pintle retaining loops are each formed from a length of warp yarn which is no greater than two and one-half repeats of the fabric weave, PA1 iii) the pintle retaining loops on the first fabric end have an "s" orientation, and PA1 iv) the pintle retaining loops on the second fabric end have a "z" orientation.
One objective of this patent was to provide a high strength seam in which 662/3% of the warp yarns are used to create the pintle loops. The key feature of MacBean is that flat monofilament warp yarns, which form the pintle and retaining loops, are re-woven back into the fabric without regard to their pre-set crimp configuration or the crimp pattern of the fabric.
The MacBean seam presents both the papermaker and paper machine clothing manufacturer with a number of practical disadvantages. First, in order to make a 662/3% loop fill seam in fabrics of this weave design, the seaming loops must be formed in pairs, thus requiring two pintle loops at one fabric end to fit into a space occupied by one retaining loop formed by one warp yarn at the second fabric end. Second, an extra manufacturing step is required to weave in the supplementary flexible strands which are needed to hold the pairs of seaming loops upright and in alignment so that the pintle can be inserted during fabric installation. Third, in order to form the disclosed two-thirds loop fill seam with its pairs of equal sized pintle loops, at least one of the warp strands of each pair must be rewoven in mis-registration with the preset crimp pattern of the fabric, thus causing an unacceptable roughness at the seam. So far as Applicant is aware, this seam design is not used commercially.
If a prior art, 50% loop fill pin seam is formed in a 3-shed weave, the result will be unsatisfactory. Although the length of warp yarn forming each pintle retaining loop is the same, the-3-shed weave pattern dictates that adjacent loop-forming yarns must each begin at different starting points in the fabric. Then, either the loops will project outwardly different distances from the fabric ends to form an irregular seam, or the warp yarns cannot be rewoven so as to maintain their crimp in registration with that of the fabric weave pattern. Thus, 3-shed and 6-shed weaves have not commonly been used in fabrics where pin seams are required, despite the usefulness of some of these weaves.
Similarly, if the prior art teachings of Lees or Penyen, for example, are applied in a 662/3% loop fill seam such as disclosed by MacBean, the result will also be unsatisfactory. Lees and MacBean contradict one another regarding the necessity of reweaving the warp yarns from the seam in registration with the fabric weave. Penven and MacBean agree insofar as both advocate the use of orthogonal pintle loops to facilitate pintle insertion. However, Penven relies upon heatsetting to reorient the inclined pintle loops into an orthogonal position, while MacBean uses supplementary yarns to achieve the same end. It will also be appreciated that neither the seam described by Penven as prior art, nor the disclosed seam, both of which are 100% loop fill seams, could be practiced in fabrics whose warp fill exceeds 50% because they would be difficult or impossible to close.
Thus, prior art seam constructions have not been entirely satisfactory in certain applications for a variety of reasons. A need still exists in the paper making industry for a dryer fabric containing a woven back pin seam which offers, in combination, the following features:
It is particularly desirable that such a seam be applicable to fabrics woven in 3-sheds, or integral multiples thereof, wherein the warp yarns forming the pintle retaining loops are rewoven in registration with the fabric weave pattern.